1. Field of the Invention
This invention relates to an apparatus for reading an image from a document, more particularly to a image sensor which can be used for example as a scanner of a facsimile.
2. Description of the Prior Art
In a scanner of a facsimile, a light emitting diode array (hereinafter, referred as "an LED array") which consists of a plurality of LEDs is generally used for irradiating a document which is moved along a path. Light beams reflected from the document are incident on an optical sensor which produces analog signals the level of which corresponds to the tone of an image on the document. The analog signals are converted into binary coded signals by an A/D converter and supplied as image signals to an image processor.
Generally, it is difficult to manufacture LEDs so as to make their optical output properties equal to each other. In most cases, therefore, an LED array has a drawback that it cannot irradiate the document uniformly. This causes the output signals of the optical sensor to be distorted so that the slice level of the binary coded signals output from the A/D converter fluctuates, resulting in uneven levels of the image signals which are obtained from images having the same tone.
In order to solve the above-mentioned problem, as shown in FIG. 2, a conventional image sensor is provided with a line memory 12 which stores digital signals supplied from an A/D converter 11, and a D/A converter 13 which converts the digital data obtained from the memory 12 into analog signals. The output signals of the D/A converter 13 are supplied as a slice level signal to the A/D converter 11.
More particularly, an LED array 16 irradiates a back plate 15 before a document 14 has moved to an irradiation region r. The light beams reflected from the back plate 15 are converted by an optical sensor 17 into electric signals which are then converted into digital signals by the A/D converter 11. The digital signals are stored in the line memory 12 as "white" information. After the document 14 has reached the region r, the light beams reflected from the document 14 are converted by the optical sensor 17 into electric signals, and the "white" information is supplied from the memory 12 to the A/D converter through the D/A converter 13, as a corrected slice level. The A/D converter 11 compares the levels of the electric signals with the corrected slice level and outputs binary coded signals as image signals.
In such a conventional image reader, however, the uneven output levels of the LEDs of the LED array 16 are not corrected, and, hence, the document 14 is unevenly irradiated by the LED array. The correction of the slice level of the A/D converter 11 is not sufficient to positively eliminate the unevenness of image signals. In other words, a conventional image sensor has a problem in that the S/N ratios of the binary coded signals are uneven.
In the conventional image reader of FIG. 2, moreover, expensive components such as the line memory 12 and the D/A converter 13 are necessary, resulting in a high manufacturing cost of the image sensor.